Amorphous metal alloy materials have become of interest in recent years due to their unique combinations of mechanical, chemical and electrical properties. The properties of amorphous metal alloy materials may be attributed to their disordered atomic structure which ensures that the material is chemically homogeneous and free from the extended defects, such as dislocations and grain boundaries, that are known to limit the preformance of crystalline materials. The amorphous state is characterized by a lack of long range periodicity, whereas a characteristic of the crystalline state is its long range periodicity.
Generally, the room temperature stability of amorphous materials depends on various kinetic barriers to the growth of crystal nuclei and on nucleation barriers that hinder the formation of stable crystal nuclei. Such barriers typically are present if the material to be made amorphous is first heated to a molten state then rapidly quenched or cooled through the crystal nucleation temperature range at a rate that is sufficiently fast to prevent significant nucleation to occur. Such cooling rates are on the order of 10.sup.6 .degree. C./second. Rapid cooling dramatically increases the viscosity of the molten alloy and quickly decreases the length over which atoms can diffuse. This has the effect of preventing crystalline nuclei from forming and yields a metastable, or amorphous phase.
Processes that provide such cooling rates include sputtering, vacuum evaporation, plasma spraying and direct quenching from the liquid state. It has been found that alloys produced by one method often cannot be similarly produced by another method even though the pathway to formation is in theory the same.
Direct quenching from the liquid state has found the greatest commercial success since a variety of alloys are known that can be manufactured by this technique in various forms such as thin films, ribbons and wires. U.S. Pat. No. 3,856,513 to Chen et al. describes novel metal alloy compositions obtained by direct quenching from the melt and includes a general discussion of this process.
The thickness of essentially all amorphous foils and ribbons formed by rapid cooling from the melt are limited by the rate of heat transfer through the material. Generally the thickness of such a film is less than 50 micrometers. This limitation on the form of synthesized amorphous metal alloys has initiated active research into other forming processes that can produce amorphous metal alloys in other shapes.
Sawmer disclosed the formation of amorphous Zr-Co alloys by a solid state reaction in a multilayer configuration, Fifth International Conference on Rapidly Quenched Metals, Wurzburg, Germany, September, 1984. Zirconium and cobalt films, having thicknesses between 100 and 500 Angstroms, were layered together and heat treated at about 180.degree. C. A diffusion process formed an amorphous Zr-Co phase at the interface of each adjacent layer.
Similarly, R. B. Schwartz and W. L. Johnson described the solid-state interdiffusion of pure polycrystalline Au and La thin films at temperatures between 50.degree. C. and 80.degree. C., "Formation of an Amorphous Alloy by Solid-State Reaction of the Pure Polycrystalline Metals", Physics Review Letters, Vol. 51, No. 5, August 1, 1983. These processes are limited to a reaction depending on the physical intimacy of two metal films.
Co-pending patent applications U.S.S.N. Ser. No. 586,380 entitled "Amorphous Metal Alloy Powders and Synthesis of Same by Solid State Decomposition Reactions" and U.S.S.N. Ser. No. 588,014 entitled "Amorphous Metal Alloy Powders and Synthesis of Same by Solid State Chemical Reduction Reactions" disclose novel processes for the obtention of amorphous metal alloys in the form of powders.
In spite of these recent advances, the widespread use of amorphous metal alloys continues to be hindered by the limited forms in which such materials are available. The need continues for new processes to synthesize amorphous metal alloys in desired shapes and forms. Especially in need is a process for the economical formation of multi-metallic amorphous metal alloy coatings.
While many amorphous metal alloys have been identified as possessing corrosion resistance to acid and base environments, none are described as coating materials since they are unavailable in such a form. U.S. Pat. No. 4,318,738 to Matsumoto et al. discloses multi-metallic carbon series amorphous alloys having corrosion resistance and taught only as powders, wires or sheets. What is needed in the field of corrosion resistant amorphous metal alloys is a commercially viable process for producing such alloys as coatings.
Multi-metallic amorphous metal alloy coatings would also have ready applications in the fields of catalytic reactions, electrochemical reactions, magnetic thin films for information storage, and metallic films for decorative and/or consumer items.
It is apparent that a low cost production process for the formation of amorphous multi-metallic alloy coatings would be a significant contribution to the field of amorphous metal alloys and their applications.
Thus, it is one object of the present invention to provide a process for the synthesis of multi-metallic amorphous alloy coatings.
It is another object of the present invention to provide novel multi-metallic amorphous alloy coatings.
It is another object of the present invention to provide novel corrosion-resistant, multi-metallic amorphous alloy coatings.
These and other objects of the present invention will become apparent to one skilled in the art from the following description of the invention and the appended claims.